To remove labels which are defined but never jumped to.

§1. inform7 tends to produce a lot of labels when compiling complicated text substitutions, but many — around 2000 in a typical run — are never branched to, either by a jump invocation or by assembly language.

These spurious labels cause no real problem except untidiness, but removing them provides a simple example of how peephole optimisation can be performed on an Inter tree. 

void EliminateRedundantLabelsStage::create_pipeline_stage(void) {
    ParsingPipelines::new_stage(I"eliminate-redundant-labels",
        EliminateRedundantLabelsStage::run, NO_STAGE_ARG, FALSE);
}

int redundant_labels_removed = 0;
int EliminateRedundantLabelsStage::run(pipeline_step *step) {
    redundant_labels_removed = 0;
    InterTree::traverse(step->ephemera.tree,
        EliminateRedundantLabelsStage::visitor, NULL, NULL, PACKAGE_IST);
    if (redundant_labels_removed > 0)
        LOG("%d redundant label(s) removed\n", redundant_labels_removed);
    return TRUE;
}

void EliminateRedundantLabelsStage::visitor(inter_tree *I, inter_tree_node *P, void *state) {
    inter_package *pack = PackageInstruction::at_this_head(P);
    if (InterPackage::is_a_function_body(pack))
        Perform peephole optimisation on this function body1.1;
}

§1.1. Perform peephole optimisation on this function body1.1 = 

    inter_symbols_table *local_symbols = InterPackage::scope(pack);
    Mark all the labels for this function as being unused1.1.1;
    Look through the function for mentions of labels, marking those as used1.1.2;
    Remove the label declarations for any that are still marked unused1.1.3;

§1.1.1. The symbol flag USED_MARK_ISYMF is free for us to use, but its value for any given symbol is undefined when we begin. We'll clear it for all labels.

Mark all the labels for this function as being unused1.1.1 = 

    LOOP_OVER_SYMBOLS_TABLE(S, local_symbols)
        if (InterSymbol::is_label(S))
            InterSymbol::clear_flag(S, USED_MARK_ISYMF);

§1.1.2. Look through the function for mentions of labels, marking those as used1.1.2 = 

    inter_tree_node *D = InterPackage::head(pack);
    EliminateRedundantLabelsStage::traverse_code_tree(D);

§1.1.3. Anything not marked used must be unused, so we can get rid of it. We do this by striking its definition; the definition of a label symbol is the line which shows where it belongs in the function (written .Example in Inter syntax). Striking this does two things: it removes the definition line; and it renders the symbol undefined. It still lives on in the function's symbols table, though, and (since we have made sure there are no references to it from anywhere) we may as well remove it.

Remove the label declarations for any that are still marked unused1.1.3 = 

    LOOP_OVER_SYMBOLS_TABLE(S, local_symbols)
        if (InterSymbol::is_label(S))
            if (InterSymbol::get_flag(S, USED_MARK_ISYMF) == FALSE) {
                InterSymbol::strike_definition(S);
                InterSymbolsTable::remove_symbol(S);
                redundant_labels_removed++;
            }

§2. The following visits every line of code in the function, in the same order it would be written out in a listing. 

void EliminateRedundantLabelsStage::traverse_code_tree(inter_tree_node *P) {
    LOOP_THROUGH_INTER_CHILDREN(F, P) {
        Examine a line of code in the function2.1;
        EliminateRedundantLabelsStage::traverse_code_tree(F);
    }
}

§2.1. If a label is used, there will be a line reading lab Example or similar. We look for such lines.

Examine a line of code in the function2.1 = 

    if (Inode::is(F, LAB_IST)) {
        inter_symbol *lab = LabInstruction::label_symbol(F);
        InterSymbol::set_flag(lab, USED_MARK_ISYMF);
    }